1. Field of the Invention
The present invention relates to apparatus and methods for sensing objects and, more specifically, to apparatus and methods for sensing objects based on their electrical permittivity or dielectric constant to obtain information about the object or its contents, such as its composition, construction, spatial characteristics and/or the like.
2. Description of the Related Art
There are many applications in which it is necessary or desirable to determine information about an unknown object, or to confirm information about an object, such as its composition, construction, spatial characteristics, and the like. This is particularly true for field applications, for example, such as at airports, public buildings, and the like, where security interests dictate that certain dangerous or potentially dangerous objects, such as weapons, explosives, etc. be detected and precluded. Interest in devices and methods for obtaining such information has increased recently, with the increased risk of terrorism and hostile acts in public places. Such devices also can be useful in detecting contraband, quality control in manufacturing, and in diagnostic medical imaging.
Devices commonly known as metal detectors are in widespread use for detecting metal objects. Metal detectors operate by radiating a low-frequency electromagnetic field, typically having an operating frequency of a few hundred Hertz (“Hz”) to a few tens of thousands of Hertz. The low-frequency magnetic fields associated with the radiated energy interact with metal objects by inducing currents on the surface of the object for reasonably good conductors, and by magnetizing the permeable ferrous materials for objects comprising ferrous components. In either case, these mechanisms produce secondary magnetic fields that can be detected in a complimentary fashion by the antenna of the metal detector device producing the primary field.
Metal detectors have been limited, for example, in their general inability to sense non-metallic objects, or objects that have relatively low magnetic permeability. In metal detectors, the physical properties that are being detected are electrical resistivity and/or magnetic permeability. Objects that have high electrical resistivity and low magnetic permeability, such as those with a high dielectric constants, accordingly do not lend themselves to satisfactory detection with metal detector-type devices.
Dielectric detection is another approach to sensing objects. In known dielectric detection devices and methods, the radiating device includes an antenna radiating electric dipole radiation, typically in the ultra high frequency (“UHF”) and super high frequency (“SHF”) radio wave region or the microwave region, e.g., in the hundreds of megahertz (“MHz”) to tens of gigahertz (“GHz”) range. Non-magnetic objects interact with this type of radiation by undergoing charge separation in the polarizable components. In this dielectric approach, the physical property of the material that is being detected by the electrical signal is the electrical permittivity or the dielectric value.
Known dielectric radio frequency (“RF”) detection devices also have been limited in a number of respects. One limitation lies in the ambiguity associated with varying sizes and shapes of the objects being measured. Existing dielectric RF detection devices operate by measuring the change in impedance of the antenna when the object is inserted into its near field. Ambiguities are caused by the fact that the size and shape of the object affect the overall impedance measured at the antenna. Such systems often are incapable of distinguishing, for example, between objects of the same size but having different dielectric constants, and objects of the same composition, and thus dielectric constant, but of different sizes and/or shapes.
Approaches have been suggested in which the frequency of the radiation used in the detection is varied, and the results are correlated with frequency changes of known dielectric samples. These approaches, however, have been ineffective in satisfactorily addressing this limitation.
Another limitation with known dielectric detection methods and apparatus lies in their sensitivity and reliability in making accurate measurements, and thus in their ability to distinguish one material from another.